1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic process such as a copying apparatus, a laser printer or a facsimile apparatus.
2. Related Background Art
An electrophotographic photosensitive member is required to have a sensitivity, electrical characteristics and optical characteristics matching an electrophotographic process to be adopted, and, particularly a photosensitive member for repeated use, of which surface is directly subjected to electrical and mechanical processes such as a charging, an image exposure, a toner development, a transfer to paper and a cleaning process, is required to have a durability capable of withstanding such processes.
In an image forming apparatus such as an electrophotographic apparatus (a copying apparatus or a printer), or an electrostatic recording apparatus, a corona charger or a roller charger is often employed as a charging apparatus for uniformly charging (including charge elimination) an image bearing member (charged member) such as an electrophotographic photosensitive member or an electrostatic recording dielectric member in a polarity and a potential required.
A corona charging method can reduce the electrical deterioration of the surface of the photosensitive member, in comparison with a roller charging method in which an AC component is contained in an applied voltage. In comparison with a corona charging in the corona charging method, a roller charging has a significantly smaller total amount of discharge products. However, in the roller charging method, a discharge current flows in a small space between the surface of the photosensitive member and the surface of the charging roller, thereby causing particles of a very high energy such as electrons or ions to repeat collisions with the surface of the photosensitive member, whereby the surface of the photosensitive member is subjected to a cleavage of molecular chains, thus being easily scrapable and damaged. Thus the surface layer of the photosensitive member is subjected to an electrical damage and a mechanical damage in case of the roller charging, but, in the corona charging utilizing a mild discharge, an electrical damage is scarce and a mechanical damage becomes predominant. Thus the corona charging method is superior for achieving a higher durability in the photosensitive member.
On the other hand, the corona charger generates a large amount of ozone products such as NOx in the primary charger (inside a shield) by the discharge of the corona charger. When the photosensitive drum is stopped and let to stand for a while after an image forming process, such ozone products react with a moisture on the photosensitive drum immediately under the charger to generate a nitrate compound or the like which remains on the drum. In a next image formation, because of an electroconductivity of such nitrate compound, the charging cannot be made to a desired potential only in an upper part of the drum that is positioned under the charger during the stopped period, thereby hindering a normal latent image formation and resulting in a phenomenon (pause memory) of a blotting or a lowered density of a line.
Therefore, for effectively discharging ozone products such as NOx generated by the discharge, and also for preventing a stain of the charger by flying toner in the apparatus, there are known methods of blowing air by a fan into the charger from the exterior of the apparatus and discharging air by an air exhaust duct from the charger to the exterior. Also in order to eliminate the influence of the conductivity of the ozone products such as NOx at a high humidity, there is often utilized a method of providing a heater within or around the photosensitive member thereby maintaining the surface of the photosensitive member equal to or higher than a predetermined temperature. In this manner the temperature fluctuation of the photosensitive member at the image formation can be significantly reduced.
In recent years, however, a higher image quality is required in color images and it becomes necessary to control the temperature of the photosensitive member with a higher precision. A potential reduction by an exposure after charging is dependent on the temperature of the photosensitive member, and a temperature change of 1° C. may induce a change of the potential as large as 2-3 V in the developed portion. In a recent color electrophotographic apparatus for which a higher stability and a higher image quality is required, a particularly stable latent image formation is essential. A time-dependent change in the contrast of the latent image should be maintained as small as possible, since it leads to a color change in the output image with an increase in the number of passed sheets.
In order to stabilize the potential in the exposed portion, there is required an improvement in the precision of the temperature of the photosensitive member in a position of a potential sensor, or a potential control capable of following the temperature change in the photosensitive member.
In order to cope with such situation, there is employed a method, as disclosed in Japanese Patent Application Laid-Open Nos. 2004-78088 and H11-265097, of detecting the temperature of the photosensitive member and executing a correction on an exposure amount or a charging potential, thereby stably controlling the potential of the exposed portion or the dark decay level which are dependent on the surface temperature of the photosensitive member.
However, in case the temperature measurement is executed in a portion of the photosensitive member easily showing a temperature fluctuation, the correcting condition may become inappropriate.
More specifically, the surface temperature of the drum is not uniform over the entire surface. For example, an external air blowing by a suction fan is required as explained in the foregoing, for example in case a corona charger is employed as the primary charger, and an air-blown portion shows a local temperature decrease during such air blowing. Also when the drum rotation is started, heat is dissipated to a low-temperature member such as an intermediate transfer member contacted at the primary transfer portion, so that a portion of the photosensitive drum after passing the primary transfer portion shows a local temperature decrease.
Therefore, in case a temperature detection for temperature control is executed in a position where the temperature becomes unstable by an external factor, a detected potential shows a fluctuation by the temperature dependence of the photosensitive member.
On the other hand, even with an improved precision in temperature, if the potential measurement is conducted in an exposed portion showing an instability in the potential, the detected potential shows a significant fluctuation by an external factor, thereby deteriorating the precision of the detected potential.